1. Field of the Invention
The present invention relates compositions and methods for providing corrosion protection to metal substrates, with bonding solutions and coating compositions for ferrous metal alloy surfaces which are free or at least substantially free of carcinogenic or toxic metals like chromate and molybdate, and to coating compositions comprising such bonding solutions and aluminum or like pigments.
2. Background Art
Compositions comprising phosphoric acid and aluminum metal are well known for use in protecting metallic surfaces such as ferrous surfaces from corrosion. In such coating compositions, particulate metallic aluminum, such as flake and/or powder, is combined with a phosphoric acid bonding solution to form a coating composition which is then applied to the metallic surface being treated. After application of the coating to the substrate, it may be heated to a first temperature, generally upwards of 500.degree. F. (260.degree. C.), until the coating is rendered essentially water insoluble. Then the coated surface may be cured at a second temperature, generally above 1000.degree. F. (538.degree. C.) to form the final protective coating.
A problem which arises in this coating process, is that when the particulate aluminum is combined with the phosphoric acid bonding solution, the acidic bonding solution can react with the aluminum. Such reactions can be very violent, causing the aluminum powder to burn or even explode, or less violent, simply resulting in the conversion of the metallic aluminum into various salts. In either case, such reactions interfere with the formation of suitable protective coatings.
Allen, U.S. Pat. No. 3,248,251, issued Apr. 26, 1966, describes coating compositions consisting essentially of a slurry of solid inorganic particulate material (such as aluminum) in an aqueous acidic bonding solution containing dissolved metal chromate, dichromate or molybdate, and phosphate. It was found that the addition of chromates or molybdates to the acidic bonding solution effectively passivated the solution toward aluminum and inhibited the oxidation of metallic aluminum, allowing particulate aluminum to be combined with the bonding solution without the undesirable chemical reaction between the acidic solution and the aluminum. These "Allen" coatings have been and still are successfully used to provide high quality coatings which protect ferrous metal alloy surfaces from oxidation and corrosion, particularly at high temperatures.
However, while chromates and molybdates have been used successfully to reduce the reactivity of the aluminum in such coating compositions, the use of chromates and molybdates has become a problem because of environmental considerations. Chromates and molybdates are generally considered to be toxic substances. Hexavalent chromium is rated as a carcinogen. Molybdenum is classified as a toxic heavy metal. It is therefore desirable to avoid the use of solutions of such salts, or at least to reduce their use. For this reason, it has become desirable to develop a phosphate/aluminum coating composition which requires little or no chromates or molybdates to control the reactivity between the acidic phosphate bonding solution and the particulate aluminum added thereto. The coating compositions should protect ferrous metal alloy surfaces from the oxidation and corrosive environmental conditions, especially at high temperatures, approximately as well as and preferably better than the so called Allen type coatings.
Recently efforts have been made to overcome the environmental problem associated with chromates and molybdates. Stetson et al., U.S. Pat. No. 5,242,488, issued Sep. 7, 1993 describes a coating composition for ferrous alloys which does not require either chromates or molybdates to control the reaction between the bonding solution and the powdered aluminum. The composition consists essentially of a slurry mixture of a bonding solution and aluminum powder. The bonding solution consists essentially of water, phosphoric acid (H.sub.3 PO.sub.4), and aluminum in an amount sufficient to substantially equilibrate the bonding solution with respect to aluminum. The bonding solution component of the coating composition of this Stetson patent requires that it contains sufficient aluminum in solution so that it is substantially equilibrated with respect to aluminum, i.e., that the amount of aluminum in solution be substantially at the saturation point and therefore, essentially inert with respect to any subsequent additions of aluminum.
The Stetson patent teaches magnesium, while not essential, may desirably be used to at least partially neutralize the aqueous phosphoric acid mixture, either before or after equilibration of the mixture with aluminum. The magnesium compound is either MgO or MgCO.sub.3. All examples given in the patent utilize magnesium ions.
An even more recent patent to Stetson, et al., U.S. Pat. No. 5,279,649, issued Jan. 18, 1994, discloses substantially the same compositions to which V.sub.2 O.sub.5 has been added to produce vanadate ion, adding another inhibitor to the aluminum equilibrated mixture. Further, in U.S. Pat. No. 5,279,650, also issued on Jan. 18, 1994, Stetson, et al. discloses a seal coating composition of the coating disclosed in the '649 companion patent which also contains ion oxide (Fe.sub.2 O.sub.3) powder. All these three coating compositions are designed to avoid the use of the chromium and molybdenum ions and require the bonding solution to be equilibrated with respect to further additions of aluminum as described in these patents. The addition of V.sub.2 O.sub.5 demonstrates the addition of a toxic substance, listed on the OSHA extremely hazardous substance list.
Although Stetson indicates that this formulation controls the reactivity between the bonding solution and the aluminum, it has been discovered in work in connection with this invention that some reaction still occurs between the bonding solution and the powdered aluminum when the slurry composition of Stetson is formulated.
It is therefore desirable to formulate a chromium and molybdenum free bonding solution, or one which is of reduced chromium and molybdenum content, which has reduced reactivity with particulate aluminum when the two are combined to form a coating composition and also to be free of toxic additives. This invention contributes to this objective.